Page 248 - Managing Global Warming

Nuclear fusion: What of the future? 207

An inertial confinement fusion energy system would need to operate at several
hertz to provide power-plant-scale electricity output. It would need to be made of sim-
ilar structural materials to a magnetic confinement plant due to the same issues with
neutron damage and activation. The plant would also require a breeder blanket to pro-
vide tritium fuel. It replaces plasma control problems with the problems of rapid
replacement of a fuel capsule in the reaction chamber, and fast laser recharge and
targeting.

5.3 Main technology challenges

Nuclear fusion presents a number of significant technological challenges, which must
be solved before it can be made commercially available. A fusion reactor is a very
complex device, and historical experimental systems have been chiefly physics exper-
iments, designed for operational flexibility and, while experimental availability is a
concern, the operating and maintenance regime required of a commercial power plant
is very different to that of an experimental device.

5.3.1 Reactor materials
Fusion materials have a twofold problem. The first is resisting radiation damage from
fusion neutrons to maintain their properties over the design lifetime—as well as expo-
sure to high temperatures and high stresses. The second is the avoidance of elements,
which form long-lived radionuclides under neutron radiation, giving rise to long-term
radioactive waste—or even changing the nature and properties of an alloy through the
production of transmutation elements. Additionally, the functional demands on mate-
rials vary widely: plasma-facing materials must be resistant against very high heat and
particle loads, without sputtering; in-vessel structural materials must be dimensionally
stable and remain ductile while installed; elements of the breeder blanket must mul-
tiply neutrons for tritium production—tritium itself is prone to permeating through
materials and cannot be allowed to leach into the environment; diagnostic and heating
systems may be directly exposed to radiation and sputtered dust, which will fog lenses
and mirrors and affect the conductivity of electronics. Some of the materials used in
reactor design and their issues are given in Table 5.1. It is difficult to conceive that a
commercial reactor will be licensable by regulatory authorities without qualification
of these materials in a nuclear environment, which implies that a high-energy neutron
source is required for materials development.

5.3.2 Power exhaust
The power used to heat the plasma, either through self-heating from alpha particles
or auxiliary systems for current drive or control, must either radiate away from
the plasma or be conducted to the vessel walls somewhere. In a tokamak (and a
stellarator), the structure designed for this is called the divertor. The divertor surface
is directly exposed to the plasma and experiences the highest heat fluxes in the device,